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Rattus norvegicus - Brown Rat

Physical Description:

The brown rat has course gray-brown fur sometimes ranging to black, with a pale underside and a long sparsely haired tail.  The total body length typically is 30-50 cm long with a tail length less than that of the body, an average from 15-25 cm.  The average weight ranges from 200-500 grams (Twigg, 1975 and Kurta, 1995).


The brown rat is found throughout Wisconsin (Kurta, 1995).  It is one of the widest spread terrestrial mammals.  The brown rat originated in East Asia and has spread throughout the world via the colonial day ships as well as newer ways of transportation like trains and planes.  The brown rat is thought to have been introduced into North America, probably from England, at about the time of the American Revolution in 1775.  The process of colonization was rapid except in the high, rocky, dry mountain regions where food shortage inhibited its spread (Twigg, 1975).  It is highly associated with human innovations like dumps, sewers and grain mills.  It will take advantage of the inside of walls amongst residential and commercial buildings.  Rats will colonize any area with an adequate supply of food as long as there is a water source near by (Ballenger, 2000 and Kurta, 1995). 


Brown rats are omnivorous and will consume almost anything they can find.  Feeding usually occurs at night and they sleep during the day.  Their primary food sources consist of grains, eggs, nuts, fruits, carrion, and discarded human food.  They also have the ability to catch fish, small rodents, and young birds for consumption.  Rats will use their mouth as a backpack and carry food to an area designated for storage and consumption.  This area is generally near the living area but may be up to a hundred yards away.  A main constraint is water unless their diet contains adequate amounts.  They are a prey for animals such as skunks, cats, snakes, and owls (Twigg, 1975, Kurta, 1995 and Ballenger, 2000).


The brown rat is polygamous and breeds year round, although fewer litters are produced in the late fall and winter months.  Females become receptive every 4-6 days and this lasts for about 24 hours.  Time of gestation is roughly 21-23 days but may be prolonged if the female is nursing an earlier litter.  The numbers of young range from 2-14, but are usually around 7.  The young are born naked and helpless.   Females will go into post birth estrus and often mate within 18 hours, easily producing at least six litters a year.  The eyes of the young will open in 14-17 days and they will be weaned around 3-4 weeks of age.  Sexual maturity is reached around the age of three months old (Kurta, 1995 and Ballenger, 2000).   Male rats contribute nothing to the raising of the young but have been noticed to deter other males form entering the area.  The life span of the brown rat is 2-3 years (Twigg, 1975).


Rats often occur in large numbers but usually go unnoticed.  Their ability to coincide with humans is leading to an increase in these numbers as the human population increases.  They are adept at hiding and scurrying as humans approach. They are often considered a menace and are the target of many pest control operations.

        During foraging rounds, they tend to use the same routes.   These routes may be several yards long or only a few feet depending on the locality of food in comparison to the nesting site.  The distance rats must travel to find food is the main determinate in home range size.  Most rats have a home range of a100-150 feet in radius but have been known to travel a mile to forage farm fields (Calhoun, 1948 and Davis, 1948).

        In larger groups there is usually a large dominate male which has his preference to water, food and nesting sites.  This male keeps other male rats from mating with his harem of females.  On the other hand females will protect their resources and will nest together.   In areas of high population, some rats low in the hierarchy will be forced to become diurnal to avoid competition for resources.  The hierarchy is based on size and weight rather than age (Calhoun, 1948).

        The main living areas of rats may be located in walls of buildings, in cracks or crevices, or they will burrow into the ground.  The hole size for burrowing rats is usually 2-3 inches in diameter.  There may be three to five entrances to the living area of a rat burrow with a dynamic design for escaping predators and natural disasters like flooding (Twigg, 1975).  The nests are built of many things like leaves, grasses, paper, insulation, garbage and anything else suitable.  These nests are used for raising the young and to provide insulation (Twigg, 1975, Ballenger, 2000 and Kurta, 1995).  Adult rats never defecate near the nests but use other areas designated for such purposes (Twigg, 1975).

        Grooming is an important part of rat behavior. Some of the basic grooming techniques are taught to the young as early as a few days of age.   Adults groom themselves carefully.  If any part of the body is abnormal the rat will gnaw at it, a procedure which tries to remove any diseased tissue.  If the skin on the tail of a rat is removed or damaged the rat may amputate its own tail and eat it. (Twigg, 1975).


Rats are host to many parasites and diseases.  It’s been said that rats have caused more deaths than all the wars.  Rats host lice, fleas, bubonic plague, spotted fever, infectious jaundice and many more diseases.  The bubonic plague may be transmitted to humans through a flea bite.  Rats can be the cause of major agricultural, home, and commercial damages due to chewing.  There have been  attempts to control the population through poisoning, trapping, and habitat destruction.  The ability of the rat to adapt to different habitats, eat any food and have successful breeding habits ensures its survival insulation (Twigg, 1975, Ballenger, 2000 and Kurta, 1995).

Literature Cited:

Ballenger, Liz.  2000, University of Michigan Animal Diversity Web.$narrative.html.

Calhoun, J. B., 1948. Mortality and Movement of Brown Rats (Rattus norvegicus) in Artificially Supersaturated Populations. Journal of Wildlife Management.  Pages 167-172.

Davis, D. E., and  Emlen, J. T., and Stokes, A. W., 1948.  Studies on Home Range in the Brown Rat.  Journal of Mammology: Pages 207-225.

Kurta, A. 1995. Mammals of the Great Lakes Region. Pages 186-188.  University of Michigan Press.  Michigan, USA.

Twigg, G. 1975. The Brown Rat.  David & Charles Inc. New Pomfret, Vermont, USA

Reference written by Joe Schwarz, Biol 378: Edited by Chris Yahnke.
Page last updated 4-29-04.

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